The invention is based on a priority application EP 04291152.9 which is hereby incorporated by reference.
The present invention relates to wireless communications systems, and more particularly, to a method for minimizing inter-cell interference in digital radio cellular communication systems employing multi-carrier transmission techniques, such as orthogonal frequency division multiplexing (OFDM).
Communication systems employing many sub-carriers, such as those that employ OFDM technology are currently used for the transmission of high-speed digital radio and television signals, e.g. Digital Audio Broadcasting (DAB) and Digital Video Broadcasting Terrestrial transmission mode (DVB-T) systems. Also, OFDM has become a widely accepted standard high bit rate transmission technique for the realization of wide-band air interfaces for wireless access to current local area networks (LAN), e.g. HiperLAN and IEEE WLAN systems. And, in the same way, the 3rd Generation Partnership Project (3GPP), for the standardization of high-speed broadband wireless mobile communication systems, is recently considering the application of OFDM techniques for the high speed data packet access (HSDPA) air interface communication between the radio access network (RAN) and the user equipment (UE).
Since OFDM is a multi-carrier transmission technique, the available spectrum is divided into many sub-carriers, each being modulated by data at a relatively low data rate. OFDM supports multiple access by allocating different sub-carriers to different users. The sub-carriers for OFDM are orthogonal and closely spaced to provide an efficient spectrum usage. Each narrow band sub-carrier is modulated using various modulation formats, such as quadrature phase-shift keying (QPSK) and quadrature amplitude modulation (QAM).
Due to the increasing popularity of high-speed broadband wireless mobile communication, of particular interest are wireless communication systems comprising bandwidth-efficient multiple access schemes. Wireless systems are shared media systems, that is, there is a fixed available bandwidth that must be shared among all the users of the system, so it is desired that radio access systems be as efficient as possible to maximize the number of users that can be served and the data rates at which the service is provided.
Typical radio access networks are implemented as so-called “cellular” systems which comprise a plurality of base stations controlled by a radio network controller (RNC), the base stations communicating with a plurality of user mobile terminals which are located inside their cell service area. It is also well known in the art that such cellular wireless systems may present particular intra- and/or inter-cell interference problems which limit the capacity of the system, the intra-cell interference being the interference experienced by one user that is caused by other users communicating within the same cell, and the inter-cell interference defined as the interference experienced by one user that is caused by other users communicating in cells other than the one in which the user is located.
In the prior art, narrow band time division multiple access (TDMA) systems, such as group special mobile (GSM), neighboring base stations use different, non-overlapping parts of the available bandwidth. However, in order to avoid substantial interference between them base stations that are sufficiently far away from each other, i.e., non-neighboring base stations, may use the same parts of the available bandwidth. Notwithstanding such frequency “re-use”, the bandwidth available for use in each cell is a small part of the total available spectrum. As a result, the system has a low “spectral” efficiency.
In the prior art, in code division multiple access (CDMA) systems, although the entire bandwidth is used by each base station which means a frequency “re-use of one”, the downlink communication in each base station cell suffers from a small number of other neighbor high-power base stations, thus limiting the capacity of the access system. This becomes particularly serious at the boundary region of a cell where the transmitting base station and an interfering base station are equidistant from the intended user. This situation is relieved by “soft handover”, where two or more base stations transmit to the user simultaneously. This soft handover mechanism though is not available for the recently standardized HSDPA service using a fast automatic repeat request (ARQ) mechanism, since ARQ is efficient only if just one base station is involved.
For OFDM, as in the CDMA case, we also consider a network deployment based on a frequency re-use scheme of one, that is, all frequencies or OFDM subcarriers, in the 5 MHz channel bandwidth, are used in every cell. In such a frequency re-use of one cellular network, the adjacent base stations also generate strong interference particularly for users at the edge of the cell coverage. Therefore, depending on the traffic loading of the surrounding cells, a basic frequency “reuse-one” OFDM network deployment may result, similar to the CDMA case, in a relatively poor quality of service and potentially low data rates for users at the cell edges.
To improve the distribution of data rate across the entire cell in OFDM access systems and to evenly distribute the interference to all users, particularly for cell edge users, some inter-cell interference mitigation techniques have been proposed. In the document “Benefits of Frequency Hopping for the OFDM DL”, 3GPP TSG-RAN1Meeting #32, R1-030523, Marne La Vallee, France, 19-23 May 2003, a solution is proposed in which the main idea is to interleave differently the transmitted subcarriers in each OFDM symbol over some or all of the channel bandwidth, prior to transmission. This variable frequency interleaving is done by simply assigning each cell (or sector) a unique hopping sequence, i.e., a distinct frequency interleaver sequence. Thus, in the case of partial frequency loading the interference is mitigated. The neighboring cells can use different frequency groups to avoid inter-cell interference. Another solution proposed to reduce inter-cell interference is described in document “Revised text proposal for OFDM Traffic Multiplexing Solutions”, 3GPP TSG RAN1 #34, R1-030970, Seoul, Korea, 6-10 Oct. 2003, which is based on special time frequency mapping patterns that simultaneously achieve large diversity gains and small cross-interference in the case of asynchronous interfering transmissions and partial frequency load.